Step 2 - Automated processing procedure

Determining positions using light‐level geolocation can be achieved using threshold methods and curve‐fitting methods (Lisovski et al. 2020). Using the light-level and time information contained in the raw logger data (a), we applied an automated processing procedure to determine, filter, and edit twilight events and calculate geographic positions using the threshold method (Figure 1). The procedure is described in detail in Bråthen et al. (2021). It further removes unrealistic positions using filters on equinox periods, speed, distribution, angle, distance, variation in timing of twilights and mid-night sun periods and smooths positions. The one aspect not automated is the calibration of sun elevation angles, crucial to estimate the latitudinal aspect of positions. It is based on a subjective, but high repeatability assessment for each annual track for each bird.

Figure 1: Conceptual illustration of information used to estimate locations from light level data using the threshold method.

The automated processing procedure yields a filtered location dataset (b) for each of the eleven SEATRACK species containing up to two locations per bird and day (one location at noon and one at midnight), with a total of 3 945 645 unique processed locations from 3 933 individuals for the eleven species tracked (Table 1).

Table 1: Summary of tracking data collected as part of SEATRACK inside and outside the Norwegian Ocean Management areas (Figure 3). Number of colonies where individuals have been tracked, as well as total number of individuals, annual tracks (often incomplete) and positions obtained over the project period.

Species	N colonies	N inds	N tracks	N pos
Black-legged kittiwake	22	981	2 267	1 111 256
Brünnich’s guillemot	16	519	1 213	521 348
Herring gull	9	57	128	66 517
Northern fulmar	16	346	892	401 803
Atlantic puffin	17	531	1 071	461 379
Common guillemot	13	523	1 252	616 103
European shag	6	248	539	215 696
Glaucous gull	4	117	233	79 584
Little auk	5	297	392	148 558
Common eider	7	276	571	280 017
Lesser black-backed gull	5	38	82	43 384
Total	55	3 933	8 640	3 945 645

Bråthen, Vegard Sandøy, Børge Moe, Françoise Amélineau, Morten Ekker, Per Fauchald, Hálfdán Helgi Helgason, Malin Kjellstadli Johansen, et al. 2021. “An Automated Procedure (V2.0) to Obtain Positions from Light-Level Geolocators in Large-Scale Tracking of Seabirds. A Method Description for the SEATRACK Project.” Report. Norwegian Institute for Nature Research.

Lisovski, Simeon, Silke Bauer, Martins Briedis, Sarah C. Davidson, Kiran L. Dhanjal-Adams, Michael T. Hallworth, Julia Karagicheva, et al. 2020. “Light-Level Geolocator Analyses: A User’s Guide.” Journal Article. Journal of Animal Ecology 89 (1): 221–36. https://doi.org/https://doi.org/10.1111/1365-2656.13036.